In a fuel cell system serving as a power source for an electric vehicle, which employs pure hydrogen gas (hereinafter referred to as hydrogen gas) for example, a circulation system for supplying the hydrogen gas has been selected so as to improve the fuel efficiency. Japanese Published Patent Application 6-275300 reports one example of this system. As a circulation method, an ejector which sucks hydrogen gas by negative pressure or a vacuum pump has been adopted.
Continuation of recirculation for hours in a circulation system sometimes causes a rise in the concentration of impurities in hydrogen gas such as nitrogen, thereby downgrading the efficiency of power generation. Also accumulation of water sometimes lowers the flow of hydrogen gas in an anode line of a fuel cell system. It is therefore necessary to conduct purging operation so as to discharge impurities such as nitrogen and water. It should be noted that the hydrogen gas of high concentration is discharged inevitably from the anode line filled with hydrogen gas during purging operation. This has posed a problem of safe hazard that the purged hydrogen gas is discharged directly into the open air.
In this connection, Japanese Published Patent Application 6-275300 does not disclose an idea that purged hydrogen gas should be discharged into the open air after undergoing a dilution process.